1. Field of the Invention
The present invention relates to a thermal recording apparatus which performs recording pixels with a predetermined density by controlling the number of pulses of a pulse train to be selectively applied to a plurality of heating elements mounted on a thermal head.
More particularly, the invention relates to a thermal recording apparatus in which the first pulse with a larger width than those of the second and subsequent pulses causes heating elements to be preheated and the second pulse causes the same to record the pixel of the minimum density, thereby efficiently heating each of the heating elements, and enabling a high contrast thermal printing even by a small number of pulses.
Furthermore, the invention relates to a thermal recording apparatus which can easily obtain substantially the same density/gradation-level curve showing the change in density with respect to each gradation level as a density/dots-area curve.
2. Description of Related Art
There have been proposed various thermal recording apparatus using a thermal head for high gradation level recording.
For example, the thermal recording apparatus using the thermal halftone gradation level recording method disclosed in Japanese patent application laid-open No. 7-156432 uses a thermal head provided with a plurality of heating resistors arranged in a row in a main-scanning direction, each heating resistor having the smaller width in a sub-scanning direction than the width in the sub-scanning direction of one pixel. This apparatus records one pixel by heating the heating resistor by a pulse train having a number of pulses with the same width, modulating the recording width in the sub-scanning direction of the pixel. It records the recording pixel of the minimum density by initial several pulses of the pulse train, and records the pixels subsequent to the first pixel of the minimum density by successively increasing the number of pulses. Since a heating resistor recording one pixel is heated by a number of pulses each having the same width, the recording start position of one pixel can be selected freely. Also, the printing start position in the sub-scanning direction of a print dot recorded by each heating resistor is shifted within a printing width of one pixel so as to be different from adjacent pixels, so that the halftone gradation level recording can be performed without deterioration in image quality such as the generation of streak noise in the main-scanning direction.
However, since the thermal recording apparatus using the above thermal halftone gradation level recording method records the pixel of the minimum density by initial several pulses of a number of pulses, which heat intermittently the heating resistor (element), it takes a long time until the heating element is heated to a predetermined temperature and causes the deterioration in heating efficiency of the heating element. In the case of the apparatus having the maximum number of pulses is small, the maximum number of gradation levels is reduced since the initial pulses are used to preheat the heating resistor. It is also difficult to adjust the density/gradation-level curve of print image to correspond to the density/dots-area curve.